1. Field of the Invention
The present invention relates to a low uniaxial anisotropy cobalt iron (CoFe) free layer structure for giant magnetoresistive (GMR) and tunnel junction heads and, more particularly, to a multilayered free layer structure wherein the uniaxial anisotropies (HK) of the layers counterbalance one another to provide a low net uniaxial anisotropy.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a nomnagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field or the lack thereof writes information in tracks on moving media, such as in circular tracks on a rotating disk.
In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to an air bearing surface (ABS) of the head and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos xcex8, where xcex8 is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor resistance changes cause potential changes that are detected and processed as playback signals by the processing circuitry.
The spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. MR coefficient is dr/R were dr is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A spin valve sensor is typically referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve.
Another type of spin valve sensor is an antiparallel (AP) spin valve sensor. The AP pinned spin valve sensor differs from the simple spin valve sensor in that the AP pinned spin valve sensor has an AP pinned structure that has first and second AP pinned layers instead of a single pinned layer. An AP coupling layer is sandwiched between the first and second AP pinned layers. The first AP pinned layer has its magnetic moment oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second AP pinned layer is immediately adjacent to the free layer and is antiparallel exchange coupled to the first AP pinned layer because of the minimal thickness (in the order of 8 xc3x85) of the AP coupling layer between the first and second AP pinned layers. Accordingly, the magnetic moment of the second AP pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first AP pinned layer.
Another type of read sensor is a tunnel junction sensor. The details of tunnel junction are described in a commonly assigned U.S. Pat. No. 5,650,958 to Gallagher et al., which is incorporated by reference herein. A typical tunnel junction sensor has two ferromagnetic layers (i.e., the pinned and free layers) separated by a thin spacer layer which relies upon the phenomenon of spin-polarized electron tunneling. The free and pinned layers, which may be NiFe or CoFe, are crystalline in structure and are separated by an electrically insulating spacer layer that is thin enough that quantum mechanical tunneling occurs between the free and pinned layers. The tunneling phenomenon is electron spin dependent, making the magnetic response of the tunnel junction sensor a function of the relative orientations and spin polarization of the conduction electrons between the free and pinned layers. Ideally, the magnetic moment orientation of the pinned layer should be pinned 90xc2x0 to the magnetic moment orientation of the free layer, with the magnetic direction of the free layer being able to respond to external magnetic fields. The pinned layer has a magnetic moment that is pinned in its orientation by exchange coupling with a pinning layer that is made of an antiferromagnetic material.
In each of the GMR sensor and the tunnel junction sensor it has been found that a thin layer of cobalt (Co), and preferably cobalt iron (CoFe), between the free layer and the spacer layer increases the magnetoresistive coefficient (dr/R) of the sensor. For purposes to be explained hereinafter the thickness of the cobalt (Co) or cobalt iron (CoFe) layer is very thin, such as 10 xc3x85, and for this reason it is sometimes referred to as a nanolayer. The nanolayer is exchange coupled to the free layer, which is typically nickel iron (NiFe). The nickel iron (NiFe) and the nanolayer are considered collectively as the free layer. Because of their exchange coupling each layer has a magnetic moment that is oriented in the same direction. This direction is parallel to the ABS in a quiescent state, namely when the sensor is not subjected to an applied field (H) from the rotating magnetic disk.
Each of the nanolayer and the nickel iron (NiFe) layer has a uniaxial anisotropy (HK). Uniaxial anisotropy is the amount of applied field (H) that is required to rotate the magnetic moment of the layer from an easy axis position to 90xc2x0 thereto. In the case of a free layer it would be the amount of applied field (H) from the rotating magnetic disk required to rotate the magnetic moment of the free layer from a position parallel to the ABS to a position perpendicular to the ABS. Nickel iron (NiFe) is a desirable material for a free layer since it has a low uniaxial anisotropy (HK). When the uniaxial anisotropy (HK) is low the magnetic moment is easily rotated by the applied field (H) from the rotating magnetic disk which makes the read sensor highly sensitive. Unfortunately, the cobalt iron (CoFe) or the cobalt (Co) of the nanolayer has a high uniaxial anisotropy (HK) which makes rotation of the magnetic moment stiff in its response to an applied field (H) from the rotating magnetic disk. The high uniaxial anisotropy of the cobalt iron (CoFe) or cobalt (Co) stiffens the response of the combined layers of the nanolayer and the nickel iron (NiFe) layer causing the free layer to be less responsive to applied fields from the rotating magnetic disk. This is why the thickness of the cobalt iron (CoFe) or the cobalt (Co) of the nanolayer is kept as thin as possible. While a thickness of 5 xc3x85 would be desirable, this is sometimes too thin and results in nonuniformity of the nanolayer. Accordingly, the thickness is generally 10 xc3x85-15 xc3x85.
Increased uniaxial anisotropy (HK) of the free layer structure also causes another problem. This problem is a decrease in the flux decay in the free layer between flux inception at the ABS and the top edge of the free layer, which distance is known in the art as the stripe height. Since the sensor is located between first and second ferromagnetic shield layers the flux leaks into the shield layers commencing at the ABS and continues to the top edge of the free layer. This leakage constitutes the aforementioned flux decay. The flux decay length along the stripe height is a length where the flux has decayed to 70% of its original amount at the ABS. Flux decay length is equal to       Flux    ⁢          xe2x80x83        ⁢    Decay    ⁢          xe2x80x83        ⁢    Length    =                    μ        ⁢                  xe2x80x83                ⁢        g        ⁢                  xe2x80x83                ⁢        t              2  
where xcexc is the permeability of the material and t is the thickness of the layer. Permeability (xcexc) is also related to uniaxial anisotropy (HK) by
xcexc=4xcfx80MS÷HK
where MS is the saturation magnetization of the material. It can be seen from the above that as the uniaxial anisotropy (HK) increases the permeability (xcexc) decreases and that when the permeability (xcexc) decreases the flux decay length decreases.
Because of the advantages of employing cobalt iron (CoFe) or cobalt (Co) as part of the free layer next to the spacer layer there is a strong-felt need for reducing the uniaxial anisotropy (HK) of the free layer structure so as to increase the sensitivity and the flux decay length of the sensor.
I have discovered a relationship between uniaxial anisotropies (HK) in a multilayered free layer structure which can be employed for providing a net uniaxial anisotropy (HK) which is low so that the magnetic moment of the free layer structure can be easily rotated by an applied field for promoting high read sensitivity of the sensor. This relationship is highly useful when a layer of the free layer structure has a high uniaxial anisotropy (HK). The relationship requires that the easy axes of a plurality of layers be set at an angle or angles with respect to one another. For instance, in a free layer structure wherein a cobalt iron (CoFe) or cobalt (Co) nanolayer is employed between the nickel iron (NiFe) free layer and a spacer layer, a second cobalt iron (CoFe) or cobalt (Co) free layer can be employed on an opposite side of the nickel iron (NiFe) free layer. The easy axes of the nanolayer between the nickel iron (NiFe) free layer and the spacer layer may be oriented parallel to the ABS, the easy axis of the nickel iron (NiFe) free layer may be oriented parallel to the ABS, and the easy axis of the cobalt iron (CoFe) or cobalt (Co) free layer on the opposite side of the nickel iron (NiFe) free layer may have an easy axis that is perpendicular to the ABS. Assuming that the nanolayer and the cobalt iron (CoFe) or cobalt (Co) free layer on the opposite side of the nickel iron (NiFe) free layer are of the same material and have the same thickness their uniaxial anisotropies will cancel leaving simply the low uniaxial anisotropy of the nickel iron (NiFe) free layer therebetween.
Many combinations of multilayered free layer structures may be made according to the present invention for lowering the net uniaxial anisotropy (HK) of the free layer structure. Another example may be simply a bilayer free layer structure wherein the first layer is cobalt iron (CoFe) and the second layer is cobalt iron (CoFe) with the first and second layers having equal thickness, the first layer having an easy axis parallel to the ABS and the second layer having an easy axis perpendicular to the ABS. In this instance the net uniaxial anisotropy (HK) would be zero and the orientation of the magnetic moment of the combined layers in a quiescent state would be responsive to an external field such as hard biasing layers at the side edges of the sensor. It should be understood that in any multilayered free layer structure the magnetic moment of each layer is oriented in the same direction since the layers are exchange coupled.
An object of the present invention is to provide a low uniaxial anisotropy (HK) free layer structure which employs a high uniaxial anisotropy (HK) material such as cobalt iron (CoFe).
Another object is to provide a multilayered free layer structure wherein the uniaxial anisotropies (HK) of the various layers counterbalance one another to provide a low net uniaxial anisotropy (HK).
A farther object is to provide a multilayered free layer structure which has an easy axis oriented parallel to the ABS and a low net uniaxial anisotropy (HK) even though one or more of the layers has a high uniaxial anisotropy (HK).
Other objects and advantages of the present invention will become apparent upon reading the following description taken together with the accompanying drawings.